1. Field of the Invention
This invention relates to an injector, such as that provided in the fuel supply system of an internal combustion engine to inject fuel supplied from a fuel tank under pressure.
2. Description of the Prior Art
In an injector comprising an electrically operated fuel valve such as that used in an internal combustion engine, a movable core (movable member) and a valve (needle valve) are generally connected together and accommodated in a fluid passage formed in a housing. Pressurized fuel from a fuel pump (via a delivery pipe) flows to a point near the valve, resides there so long as the valve is kept in the closed position by the force of a spring, and is injected or jetted from a nozzle at the tip of the injector when the valve is retracted by energization of a solenoid.
In this conventional injector only the valve located downstream (toward the tip side) is borne by the bearing surface of the fluid passage and the movable core located upstream does not contact the surface of the fluid passage. This portion is instead structured as a bearing (guide). This allows fuel to flow between the fluid passage surface and the movable core to as far as the valve. The response is therefore good since the fluid resistance encountered is low. However, the bearing structure between the valve and the bearing surface has to be machined to high precision. If it is not, the durability of the injector is degraded owing to valve tilting that occurs owing insufficient support of the load acting on the valve.
Japanese Laid-Open Patent Application No. Hei 2(1990)-66,380 (claiming the priority of German Patent Application No. P3825135.3 on Jul. 23, 1988) teaches an injector to be used for an internal combustion engine in which the movable core also has a bearing structure. Specifically, as shown in FIG. 4 of this publication, a guide flange 38 made of a non-magnetic substance is provided adjacent to a fixed core (sleeve) to serve as a bearing for the movable core.
FIG. 10 of this specification of the subject application illustrates this prior art structure. As shown, the movable core is retained with a slight gap of several .mu.m to ten and several .mu.m between its full peripheral surface and the guide flange 38.
The fuel (gasoline, methanol or the like) passing through the fluid passage is an incompressible fluid. Therefore, as shown in FIG. 10, operation of the valve produces a volume change throughout the fluid in the fluid passage corresponding to the valve movement (stroke).
The valve is pulled or lifted in the upstream direction against the fluid pressure. However, since only a slight clearance is present between the movable core and the guide flange 38 in this prior art fuel valve, a long time is needed for the volume change corresponding to the stroke to be completed. The response is therefore poor owing to this prolongation of the operation time. This makes it difficult to increase the amount of fuel injected per unit time. The severity of this problem increases with increasing fuel pressure.
On the other hand, in the first-described conventional injector arrangement in which the movable core does not have a bearing structure, the response is good but problems arise regarding machining precision and durability.